The present invention relates to an electrode return control method for an electric spark machine.
Electric spark machines include wire-cut types in which a wire electrode is moved relative to a work to be cut along an instructed path, and types in which the electrode itself has a specific outer configuration and a work is machined to a configuration corresponding to the outer configuration of the electrode by moving the latter relative to the work.
FIG. 1 illustrates a machining operation of the latter type electric spark machine, in which an electrode 2 serving as a punch is supported by a spindle 3 and advanced in the arrow direction by a servo motor (not shown) to cut the work 1 to the shown configuration. A pulse voltage is applied from a power source 4 between the work 1 and the electrode 2, the work serving as a die. As a result, electric sparks are produced in a small gap between the electrode 2 and the work 1 upon which the latter is machined. By advancing the electrode 2 relative to the work 1 the latter is further machined corresponding to the outer configuration of the electrode 2. The region of the work 1 which is to be machined can be enlarged easily by controlling the pulse voltage or energy to be supplied to the gap or by moving the electrode 2 eccentrically with respect to the work 1.
In such spark machine as mentioned above, there may be a short-circuit when the electrode 2 comes in contact with the work 1 due to material particles removed from the work 1. In such case, the electrode 2 is moved away from a point at which the short-circuit occurs so that the short-circuit condition disappears and the material particles can be removed from the point in question. After these procedures are completed the electric spark machining operation is restarted.
In order to realize these procedures, it has been usual to return the electrode 2 along the path along which the preceding machining operation was performed. As another method for moving away the electrode, a point is preliminarily set and upon the occurrence of a signal indicative of such short-circuit the electrode is automatically returned to the preset point along the shortest path to the latter. This method is disclosed in Japanese Kokai No. 51021/1983.
According to the former method, when the electrode has a circular cross section and is advanced with respect to the work 1 to machine a round recess as shown in FIG. 2a, or when the electrode has a square cross section and is advanced along a rectangular path to machine a rectangular recess as shown in FIG. 2b, it is very difficult to remove a short-circuit condition if the electrode 2 is returned along the machining path, because the gap between the work 1 and the electrode 2 being returned is very close to the gap in which the short-circuit occurred.
In the preset point system disclosed in the afore-mentioned Japanese Kokai the electrode 2 is returned to a preset point A upon the occurrence of a short-circuit condition as shown in FIG. 2c, wherein the distance between the preset point A and the position of the electrode at which the short-circuit occurs increases with the depth of machining. That is, the return distance of the electrode 2 increases with the progress the machining, resulting in an increased time required for purposes other than actual machining. Consequently, the total machining time is increased. It may be possible to resolve this increased machining time by programming the machining process such that the preset point itself is updated with the progress of machining so as to make the amount of returning of the electrode substantially constant. However, it is clear that such programming is very complicated, causing other problems to appear.